Modern diagnostic medicine has benefitted significantly from radiology, which is the use of radiation, such as x-rays, to generate images of internal body structures. In general, to create an x-ray image, x-ray beams are passed through the body and absorbed, in varying amounts, by tissues in the body. An x-ray image is created based on the relative differences in the transmitted x-ray intensities.
FIG. 1A is a diagram illustrating a fluoroscopic C-arm x-ray imaging device. Imaging device 100 includes C-arm 103 attached to mobile base 102. X-ray source 105 is located at one end of C-arm 103 and x-ray receiving section 106 is located at the other end of C-arm 103. Receiving section 106 generates an image representing the intensities of received x-rays. Typically, receiving section 106 comprises an image intensifier that converts the x-rays to visible light and a charge coupled device (CCD) video camera that converts the visible light to digital images.
Images taken at the mobile base 102 are transmitted to control unit 120 for analysis. In particular, control unit 120 typically provides facilities for displaying, saving, digitally manipulating, or printing a hard copy of the received images. Control unit 120 additionally includes controls for controlling base unit 102.
In operation, the patient is positioned in area 110, between the x-ray source 105 and the x-ray receiving section 106. In response to an operator's command input at control unit 120, x-rays emanating from source 105 pass through patient area 110 and into receiving section 106, which generates a two-dimensional image of the patient.
Although each individual image taken by base unit 102 is a two-dimensional image, techniques are known in the art through which multiple two-dimensional images taken from multiple perspectives can be used to infer the three-dimensional location of an anatomical projection. To change image perspective, C-arm 103 rotates as shown, for example, in FIG. 1B. By taking multiple two-dimensional images of point 124, but from different perspectives, the three-dimensional position of point 124 may be determined.
Raw images generated by receiving section 106 tend to suffer from undesirable distortion caused by a number of factors, including inherent image distortion in the image intensifier and external electromagnetic fields. An example of a true and a distorted image is shown in FIG. 2. Checkerboard 202 represents the true image of a checkerboard shaped object placed in image taking area 110. The image taken by receiving section 106, however, suffers significant distortion, as illustrated by distorted image 204.
Intrinsic calibration, which is the process of correcting image distortion in a received image and learning the projective geometry of the imager, involves placing "calibration markers" in the path of the x-ray, where a calibration marker is an object opaque to x-rays. The calibration markers are rigidly arranged in predetermined patterns in one or more planes in the path of the x-rays and are visible in the recorded images.
Because the true relative position of the calibration markers in the recorded images is known, control unit 120 is able to calculate an amount of distortion at each pixel in the image (where a pixel is a single point in the image). Accordingly, control unit 120 can digitally compensate for the distortion in the image and generate a distortion-free, or at least a distortion improved image. A more detailed explanation of a method for performing intrinsic calibration is described in U.S. Pat. No. 5,442,674 to Picard et al, the contents of which are incorporated by reference herein.
A notable disadvantage in the conventional method of compensating for image distortion, as described above, is that although there is significantly less distortion in the image, projections of the calibration markers are present in the image. This is undesirable, as the projections of the markers may occlude important portions of the patient's anatomy and/or act as a visual distraction that prevents the clinician from concentrating on important features of the image.
There is, therefore, a need in the art to improve the intrinsic calibration process.